Methods For Forming Composite Articles From Tailorable Polyimide Resin Systems using RTM and RI Techniques

ABSTRACT

A method for forming a polyimide composite article utilizes a polyimide resin system including at least a first prepolymer component and a second prepolymer component. A preform structure is tackified with the first prepolymer component. Using resin infusion or resin transfer molding techniques, the tackified preform structure is contacted with the second prepolymer component. The polyimide resin system is cured under suitable cure conditions so that the first and second prepolymer components mix and react to produce the polyimide composite structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of application Ser. No.11/928,274 filed Oct. 30, 2007, which is a Continuation in Part ofapplication Ser. No. 11/757,683 filed Jun. 4, 2007, which is aContinuation-in-Part of application Ser. Nos. 11/383,079, filed May 12,2006; 11/383,086, filed May 12, 2006; 11/383,092, filed May 12, 2006;11/383,100, filed May 12, 2006; 11/383,104, filed May 12, 2006; all ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

This invention relates generally to resin transfer molding (RTM) andresin infusion (RI) methods applicable to polyimide resin systems, resinsystems processible using RTM and RI methods, and to reinforcedcomposite articles obtained therefrom.

Fiber-reinforced composite materials, which are made up of reinforcingfibers and matrix resins, are lightweight and may exhibit excellentmechanical properties. As such, these composite materials have beenwidely used in a variety of structural and non-structural applicationsin the aerospace industry.

Various methods or techniques, such as prepreg, hand lay-up, filamentwinding, pull-trusion, RTM and RI, have been used to produce reinforcedcomposite materials. In the RTM method, a preform structure made up ofreinforcing material is placed in a mold, a resin poured therein toimpregnate the preform, and the impregnated preform structure cured toproduce a molded product. The RTM method offers the advantage that alarge component having a complicated shape can be molded in a shortperiod of time.

The preform structure may include a tackifier which, when heated, willfuse onto the surface of the reinforcing material and then solidify uponcooling. Layers of the reinforcing materials with the tackifier can bestacked together, the tackifier heated, fusing the plies together underan appropriate pressure, and then cooled to form the net-shaped preformstructure. The multilayered preform structure is placed into a mold, thematrix resin added, and the composite formed using usual resin transfermolding processes. U.S. Pat. No. 5,766,534 discloses a process forpreparing a matrix resin composite utilizing a preform comprising two ormore layers of reinforcing material and a tackifier of a curable resinapplied to at least one layer of a reinforcing material. The layeredassembly is compressed while the tackifier is at least partiallycrosslinked. Thereafter, the preform is contacted with the matrix resin,which may be the same or different from the tackifier resin. The matrixresin and tackifier are finally cured to form the matrix resincomposite.

As disclosed in U.S. Pat. No. 7,129,318, the use of composite materialshaving polyimide resin matrices is increasing because of theirlightweight and load-bearing characteristics and their oxidativestability at elevated temperatures. However, polyimide resin systemspresent challenges for use with RTM and RI techniques. Fiber-reinforcedcomposite materials that use polyimide resins as the matrix resin aregenerally prepared using prepreg methods. For example, poly(amid) acidsolutions may be processed into prepreg with various reinforcing fibers.The poly(amide) acid solutions have low solids content and highviscosity, presenting processing problems. This material is thenhand-laid into composites in a labor-intensive operation.

Polyimide resin systems for use in RTM processes generally usepreimidized polyimides with molecular weights ranging from 800 to 1100g/mol. The preimidized powder may be melted and injected into a dryfiber preform. However, current RTM polyimide parts suffer frommicrocracking, poor thermal stability, or offer only limited temperaturecapability. The low viscosity/low molecular weight needed for injectionoften does not create favorable end properties. There currently does notexit a tackifier system suitable for use with polyimides.

Accordingly, it would be desirable to provide an RTM and RI methods thatutilize the benefits of a tackifier, suitable for polyimide resinsystems that provide reinforced composite structures that exhibit goodmechanical an thermal properties.

BRIEF DESCRIPTION OF THE INVENTION

The above-mentioned need or needs may be met by exemplary embodimentswhich provide a methods for forming polyimide composite structures. Anexemplary method includes tackifying a preform structure with a firstprepolymer component; contacting the tackified preform structure with asecond prepolymer component, wherein a polyimide resin system iscomprised of the first prepolymer component and the second prepolymercomponent; and curing the polyimide resin system under suitableconditions such that the first and second prepolymer componentsintermingle and react to provide a polyimide composite structure.

An exemplary method includes tackifying a preform structure with a firstprepolymer component comprising at least one of a monomeric blend or areaction product of an end-capping component, at least one dianhydrideor derivative thereof, and at least one diamine. Using resin infusion orresin transfer molding techniques, the tackified preform structure iscontacted with a second prepolymer component, different from the firstprepolymer component, including at least one of a monomeric blend or areaction product of an end-capping component, at least one dianhydrideor derivative thereof, and at least one diamine. A polyimide resinsystem is comprised of the first prepolymer component and the secondprepolymer component. The polyimide resin system is cured under suitableconditions such that the first and second prepolymer components mixedand react to provide a polyimide composite structure.

An exemplary embodiment includes a method comprising: providing apolyimide resin system comprising an amount of a first prepolymercomponent and an amount of a second prepolymer component; providing apreform structure comprising reinforcing material; tackifying thepreform structure with at least a portion of the first prepolymercomponent; contacting the tackified preform with the second prepolymercomponents and a remainder of the first prepolymer component; and curingthe tailorable polyimide resin system under suitable conditions suchthat the first and second prepolymer components intermingle and react toprovide a polyimide composite structure.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments disclosed herein provide polyimide systems thatsimultaneously offer low toxicity, a high glass transition temperature,excellent thermal oxidative stability, and the ability to be processedusing RTM and RI methods. Furthermore, embodiments disclosed hereinprovide tailorable polyimide systems wherein relative amounts ofstarting materials may be altered to achieve desired outcomes.

In an exemplary embodiment, a polyimide matrix of a reinforced compositearticle is the reaction product of a mixture of monomeric reactants,polyimide-precursor reaction products, oligomers, and mixtures thereof.Exemplary embodiments include composite articles formed by RTM or RImethods.

Exemplary embodiments include a pre-polymer polyimide resin system thatincludes a first prepolymer component and a second prepolymer component.The first prepolymer component may be a mixture of monomers, a mixtureof oligomers, or a pre-imidized component. The second prepolymercomponent may be a mixture of monomers, a mixture of oligomers, or apre-imidized component. The first and second prepolymer components arecapable of reacting to provide the cured polyimide matrix for acomposite article.

An exemplary first prepolymer component includes a first monomericmixture, or repeat units from monomers including at least oneend-capping agent, at least one aromatic dianhydride or a derivativethereof (e.g., the ester product formed from the dianhydride andalcoholic solvent), and at least one diamine. The second prepolymercomponent includes a second monomeric mixture, or repeat units frommonomers including at least one end-capping agent, at least one aromaticdianhydride, and at least one diamine. The at least one aromaticdianhydride or the at least one diamine in the first prepolymercomponent is different from the at least one aromatic dianhydride orderivative thereof, or the at least one diamine in the second prepolymercomponent. The selection of the end-capping agent(s), aromaticdianhydrides, diamines, and their relative molar ratios, are consideredwith respect to the desired property outcomes such as molecular weight,processibility, high temperature performance, and the like.

End-group components may include structures that are capable of formingoligomer compounds and capable of crosslinking in an additionpolymerization reaction to form a crosslinked polyimide structure.Crosslinkable-group-containing end blocking agents of various kinds areusable depending on the synthesis process of the polyimide, includingmonoamines and dicarboxylic acid anhydrides as representative examples.A variety of crosslinkable groups may be selected in accordance withmolding or forming conditions.

The crosslinkable group structures contained in the end groups mayinclude ethynyl groups, benzocyclobuten-4′-yl groups, vinyl groups,allyl groups, cyano groups, isocyanate groups, nitrilo groups, aminogroups, isopropenyl groups, vinylene groups, vinylidene groups, andethynylidene groups.

The above described, crosslinkable-group-containing end blocking agentscan be used either singly or in combination. Some or all of the hydrogenatoms on one or more of the aromatic rings of the end group containingmaterial may be replaced by a like number of substituent groups selectedfrom halogen groups, alkyl groups, alkoxy groups, and combinationsthereof.

Exemplary end group components may include, but are not limited to, thefollowing end group structures:

nadic end groups, including, but not limited to the following formula:

vinyl end groups including, but not limited to the following formula:

acetylene end groups including, but not limited to the followingformula:

phenylethynyl end groups including, but not limited to the followingformula:

and mixtures thereof.

Ar as shown above in the nadic and phenylenthynyl end group structuresmay include aromatic groups, such as substituted or unsubstitutedaromatic monocyclic or polycyclic linking structures. Substitutions inthe linking structures may include, but are not limited to ethers,epoxides, amides, esters and combinations thereof.

The dianhydride component may include, but is not limited to, monomershaving an anhydride structure, wherein an exemplary structure includes atetracarboxylic acid dianhydride structure. The dianhydride componentemployed may be any suitable dianhydride for forming crosslinkable orcrosslinked polyimide prepolymer, polymer or copolymer. For example,tetracarboxylic acid dianhydrides, singly or in combination, may beutilized, as desired.

Illustrative examples of aromatic dianhydrides suitable for use include:2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane dianhydride;4,4′-bis(3,4-dicarboxyphenoxy) diphenyl ether dianhydride;4,4′-bis(3,4-dicarboxyphenoxy)diphenyl sulfide dianhydride;4,4′-bis(3,4-dicarboxyphenoxy)benzophenone dianhydride;4,4′-bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride;2,2-bis(4-(2,3-dicarboxyphenoxy)phenyl)propane dianhydride;4,4′-bis(2,3-dicarboxyphenoxy)diphenyl ether dianhydride;4,4′-bis(2,3-dicarboxyphenoxy)diphenyl sulfide dianhydride;4,4′-bis(2,3-dicarboxyphenoxy)benzophenone dianhydride;4,4′-bis(2,3-dicarboxyphenoxy) diphenyl sulfone dianhydride;4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy) diphenyl-2,2-propanedianhydride; 4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)diphenylether dianhydride;4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)diphenyl sulfidedianhydride;4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)benzophenonedianhydride and4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)diphenyl sulfonedianhydride, 1,2,4,5-benzenetatracarboxylic dianhydride as well asmixtures comprising one of the foregoing dianhydrides.

Exemplary dianhydride components include the following dianhydridecompounds:

3,4,3′,4′-biphenyltetracarboxylic dianhydrides (BPDA) having thefollowing formula:

3,4,3′,4′-benzophenonetetracarboxylic dianhydrides (BTDA) having thefollowing formula:

2,2-bis(3′,4′-dicarboxyphenyl) hexafluoropropane dianhydrides having thefollowing formula:

pyromellitic dianhydrides having the following formula:

and mixtures thereof.

Depending on the fabrication process, tetracarboxylic acidmonoanhydrides, tetracarboxylic compounds other than anhydrides, ortheir derivatives such as salts may also be used as desired instead ofthe above-recited dianhydrides. The dianhydride components, as describedabove, may be used either singly or in combination as needed.

The aromatic dianhydrides can be prepared by any suitable fabricatingmethod known in the art. One suitable fabrication method for fabricatingaromatic dianhydrides may include hydrolysis, followed by dehydration,of the reaction product of a nitro substituted phenyl dinitrile with ametal salt of dihydric phenol compound in the presence of a dipolar,aprotic solvent.

The diamine component may include, but is not limited to, an aromaticdiamine monomer having the following formula:

H₂N—Ar—NH₂

Ar as used in this formula preferably includes aromatic compounds,including substituted aromatic compounds and compounds having multiplearomatic rings. Substituent groups for substitution in the Ar group mayinclude any suitable functional group, including, but not limited tohalogen groups, alkyl groups, alkoxy groups, and combination thereof.

Examples of suitable diamine components may include, but are not limitedto: 1,3-bis(aminophenoxy)benzene, 1,4-bis(aminophenoxy)benzene,1,4-phenylenediamine (“para-PDA” or “pPDA”), 1,3-phenylene diamine(“meta-PDA” or “mPDA”), 4,4′-[1,3-phenylenebis(1-methyl-ethylidene)]bisaniline (“Bis aniline M” or “Bis-M”),ethylenediamine, propylenediamine, trimethylenediamine,diethylenetriamine, triethylenetetramine, hexamethylenediamine,heptamethylenediamine, octamethylenediamine, nonamethylenediamine,decamethylenediamine, 1,12-dodecanediamine, 1,18-octadecanediamine,3-methylheptamethylenediamine, 4,4-dimethylheptamethylenediamine,4-methylnonamethylenediamine, 5-methylnonamethylenediamine,2,5-dimethylhexamethylenediamine, 2,5-dimethylheptamethylenediamine,2,2-dimethylpropylenediamine, N-methyl-bis(3-aminopropyl) amine,3-methoxyhexamethylenediamine, 1,2-bis(3-aminopropoxy)ethane,bis(3-aminopropyl) sulfide, 1,4-cyclohexanediamine,bis-(4-aminocyclohexyl)methane, m-phenylenediamine, 2,4-diaminotoluene,2,6-diaminotoluene, m-xylylenediamine, p-xylylenediamine,2-methyl-4,6-diethyl-1,3-phenylene-diamine,5-methyl-4,6-diethyl-1,3-phenylene-diamine, benzidine,3,3″-dimethylbenzidine, 3,3″dimethoxybenzidine, 1,5-diaminonaphthalene,bis(4-aminophenyl)methane,bis(2-chloro-4-amino-3,5-diethylphenyl)methane, bis(4-aminophenyl)propane, 2,4-bis(b-amino-t-butyl) toluene,bis(p-b-amino-t-butylphenyl)ether, bis(p-b-methyl-o-aminophenyl)benzene,bis(p-b-methyl-o-aminopentyl)benzene, 1,3-diamino-4-isopropylbenzene,bis(4-aminophenyl) sulfide, bis(4-aminophenyl) sulfone,bis(4-aminophenyl)ether, 1,3-bis(3-aminopropyl) tetramethyldisiloxaneand mixtures comprising at least one of the foregoing organic diamines.

Further, these diamines are also usable in place of some or all of thehydrogen atoms on one or more of the aromatic ring(s) of each of thediamines. A like number of ethynyl groups, benzocyclobuten-4′-yl groups,vinyl groups, allyl groups, cyano groups, isocyanate groups, nitrilogroups and/or isopropenyl groups, which can act as crosslinking points,may also be introduced as substituent groups on the aromatic rings,preferably to an extent not impairing the moldability or formability.

Glass transition temperature (Tg) is a measure of the ability of thepolymer to maintain properties at elevated temperatures. Because bulkmotion of the polymer is restricted below the Tg, the higher the Tg amaterial displays, typically, the higher the temperature capability ofthat material. Therefore, Tg of the crosslinked polyimide matrix may bea driving consideration in the make up of the prepolymer blend.

Melt viscosity is a measure of a fluids resistance to flow attemperatures above the melt point. For processing composites, it isgenerally desirable to have melt viscosities below 100,000 centipoise(cps) with the preferred range or 40,000 cps-800 cps wherein the meltviscosity is dependent upon the processing utilized. If the meltviscosity is not sufficiently low, processing requires excessivepressures in order to make the resin flow. Lower melt viscositiesgenerally lead to greater processing options due to decreased pressureneeds. Thus, a desired melt viscosity of the prepolymer blend mayinfluence the respective amounts of the components in the prepolymerblend.

Thermal Oxidative Stability (TOS) is the ability of the polymer towithstand elevated temperatures in an oxygen-containing environment,such as air, with minimal loss of weight and/or properties. Turbineengine components often operate in high pressure as well as hightemperature environments and the high pressure acts to increase theconcentration of oxygen accelerating the deterioration of compositeproperties. Since, in a composite, compression strength is aresin-dominated property, the retention of compression strength afterlong-time exposures to high temperatures is monitored as a measure ofTOS. Weight loss over time is also used as a measure. Polymers degradethrough mechanisms, such as volatilization, resulting in a compositehaving reduced mass due to this loss of polymer. One test used herein tomeasure TOS includes placing a plaque of polymeric or composite materialin a chamber, increasing the temperature and pressure within the chamberto a predetermined temperature and pressure, and holding theseconditions for up to 150 hrs with multiple atmospheric changes over thecourse of the test. The plaques are then removed and tested for weightloss and retention of compression strength. The weight loss andretention of compression strength reflect service conditions in aturbine engine and provide a measure of the longer-term stability of thepolymer material. A higher TOS is important for material that will beplaced in a high temperature environment for long periods of time. Thecrosslinked polyimide copolymer preferably has a TOS of less than about2.0% weight loss.

One embodiment includes utilizing the prepolymer blends in a resininfusion (RI) process. In RI, a fiber containing preform is typicallyplaced on a mold or other surface capable of providing the curedmaterial with the desired geometry. A preferred fiber, particularly foraerospace applications, is carbon fiber. The fiber reinforcement of thepreform is not limited to carbon fiber and may include any suitablefiber having high strength, sufficient stiffness, and relatively lowdensity. The fiber for impregnation may be a fiber in any suitable formincluding, but not limited to uniaxial, braided, multi-layered, or wovenforms. In addition, the fibers may be continuous fibers, chopped fiber,braided fiber, fiber fabric, woven fibers and noncrimp fabric, unitapefiber, fiber film or any suitable form of fiber that results in areinforced composite material when cured. In addition, multiple types offibers may be utilized in the preform.

An exemplary prepolymer blend may be placed as a film layer or layers onor within intermediate layers of the reinforcing fiber preform to coverall or a majority of the preform. Alternatively, a film material,including the prepolymer blend, may be provided as at least a portion ofthe preform, wherein the material provided includes fibers onto whichthe resin blend has been placed into contact. The prepolymer blend resinmaterial may be applied onto the entire surface of the reinforcing fiberpreform. Alternatively, the matrix material may be interleaved betweenlayers of the preform to cover all the layers of reinforcing fiberpreform. Sufficient prepolymer material is provided to impregnate thepreform during a heated resin infusion phase. Typically, the RI methodwill include placing a barrier layer, such as a polytetrafluoroethylenebarrier onto the prepolymer blend and/or prepreg material to assist incontrolling the flow of resin. The perform and prepolymer blend may thenbe placed into a vacuum membrane or similar vacuum providing apparatus.The mold, fiber, resin, barrier layer and vacuum membrane may be placedinto an autoclave or other controlled atmosphere device. The preciseprocessing parameters utilized can vary and may depend upon theparticular materials used as the first and second prepolymer componentsin the prepolymer blend.

In one embodiment, the temperature and pressure are increased within theautoclave, while simultaneously drawing a vacuum on the vacuum membrane.The increased temperature and vacuum facilitate the infiltration of theresin into the preform. The temperature and vacuum are maintained untilthe resin has sufficiently impregnated the preform to avoid theformation of voids. After infiltration, the temperature may be increasedto begin crosslinking of the prepolymer blend. The specific parametersof the cure cycle vary and depend upon the particular materials used asthe first and second prepolymer components in the prepolymer blend.

In another embodiment, the polyimide prepolymer blend may be processedusing resin transfer molding (RTM). The materials utilized for the fiberreinforcement and the matrix are substantially the same as those used inthe discussion of the RI process above. However, in RTM, an injectionsystem is utilized to inject the prepolymer mixture into a mold bypressurization of the prepolymer mixture. The mold, which has thesubstantial geometry of the finished component, includes the fiberpreform. The pressurized prepolymer blend impregnates the dry fibers ofthe fiber preform and is cured to crosslink the prepolymer mixture andform the final component. The specific parameters of the cure cycle varyand depend upon the particular materials used as the first and secondprepolymer components in the prepolymer blend.

In some cases, the desired prepolymer blend (i.e., a blend that willprovide desired qualities in a composite article) may not be amenable toconventional RTM or RI processing methods. Exemplary embodimentsdisclosed herein provide methods for obtaining the desired compositearticle properties, while utilizing RTM or RI techniques.

In an exemplary embodiment, the desired prepolymer blend includes firstand second prepolymer components, which when suitably combined and curedwill provide the desired composite article. For example, embodimentsdisclosed herein provide for use of one of the first or secondprepolymer component as a tackifier for an RI or RTM method. Forexample, the preform structure may be impregnated with a suitable amountof, for example, the first prepolymer component. The first prepolymercomponent may have a greater molecular weight than desired forconventional RI or RTM processing and thus, standing alone, may not besuitable for use with RI or RTM techniques. However, the firstprepolymer component may impart desired qualities to the fully curedpolyimide composite article. In order to incorporate the desiredqualities into the composite article, while employing RTM or RItechniques, the first prepolymer component is utilized as a tackifierfor the preform structure. The first prepolymer component may be amixture of monomers, a blend of oligomers, or a pre-imidized reactionproduct.

The preform structure, tackified with the first prepolymer component, isthen infused with a suitable amount of the second prepolymer componentas in conventional RTM or RI processes. The first and second prepolymercomponents mix during processing and react under suitable reactionconditions to provide a polyimide composite article including acrosslinked polyimide matrix supported by the preform. In this example,the polyimide composite article may exhibit enhanced properties (i.e.,Tg, void content, thermal oxidative stability, tensile strength) due tothe incorporation of the first prepolymer component. Of course, othercombinations of prepolymer components may be utilized following theprinciples taught herein. For example, the second prepolymer componentcould be used as the tackifier, and the first prepolymer componentinfused into the preform structure. In other exemplary embodiments, thefirst and second prepolymer components may themselves be blends ormixtures of pre-polyimide components.

In an exemplary embodiment, a prepolymer blend includes a firstprepolymer component comprising at least a first polyimide oligomerhaving the formula E₁-[R₁]_(n)-E₁; and a second prepolymer componentselected from the group consisting of M₁, a second polyimide oligomerhaving the formula E₂-[R₂]_(n)-E₂, and combinations thereof; wherein R₁and R₂ independently comprise the following structure:

wherein n comprises from about 1 to about 5, wherein V is a tetravalentsubstituted or unsubstituted aromatic monocyclic or polycyclic linkingstructure, R is a substituted or unsubstituted divalent organic radical,E₁ and E₂ independently comprise crosslinkable functional groups, andwherein M₁ comprises a mixture of monomeric compounds including adiamine component comprising at least one diamine compound, adianhydride component comprising at least one dianhydride compound, andan end group component comprising at least one end group compound.

Exemplary properties of the prepolymer blend, or the crosslinkedpolyimide matrix that may be varied include imidization temperature,maximum cure temperature, molecular weight distribution, tack, drape,ability to process using film infusion, ability to process using RTM,ability to modify the prepolymer blend with fillers or other agents,tensile strength, compression strength, inplane shear, and wetproperties.

In other embodiments, prepolymer blends may include a plurality ofpreimidized reaction products. The preimidized reaction products may beblended in various ratios to optimize desired outcomes.

Using the processes described above, prepolymer blends can be readilytailored to provide desired property outcomes in the blends and thecrosslinked matrices.

EXAMPLE

A prepolymer mixture was formed from a blend of dimethyl ester of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (“BTDA”),(4,4′-[1,3-phenylene bis(1-methyl-ethylidene)]bisaniline) (“Bis AnilineM”), paraphenylene diamine (“para PDA”), norbornene 2,3-dicarboxylicacid (“NE”) and 3,3′,4,4′-biphenyl-tetracarboxylic dianhydride (BPDA).The above blend was further mixed with a solid powder second prepolymercomponent having a reaction product of NE, BTDA, metaphenylene diamine(meta PDA), and Bis-Aniline M.

The liquid prepolymer component included the following molarcompositional concentrations of monomers:

30 mol % Bis Aniline M,

12.9 mol % p PDA,

28.6 mol % NE and

varying mol % of BPDA and BTDA, as shown in TABLE 1, wherein the totalmol % of the combination of BPDA and BTDA is 28.5 mol %.

TABLE 1 MOLAR COMPOSITIONS OF EXAMPLES 1-12 Bis Aniline Example BTDABPDA M p PDA NE 1 24.2% 4.3% 30.0% 12.9% 28.6% 2 24.2% 4.3% 30.0% 12.9%28.6% 3 24.2% 4.3% 30.0% 12.9% 28.6% 4 21.4% 7.1% 30.0% 12.9% 28.6% 521.4% 7.1% 30.0% 12.9% 28.6% 6 21.4% 7.1% 30.0% 12.9% 28.6% 7 24.2% 4.3%30.0% 12.9% 28.6% 8 24.2% 4.3% 30.0% 12.9% 28.6% 9 24.2% 4.3% 30.0%12.9% 28.6% 10 21.4% 7.1% 30.0% 12.9% 28.6% 11 21.4% 7.1% 30.0% 12.9%28.6% 12 21.4% 7.1% 30.0% 12.9% 28.6%

A solid powder prepolymer component was added to the liquid monomermixture in Examples 1-12. The solid powder prepolymer component includeda reaction product of the following components:

40 mol % NE,

20 mol % BTDA,

28 mol % 1,3-phenylene diamine (meta PDA), and

12 mol % bis-aniline M.

The reaction product forming the solid powder prepolymer component was apolyimide oligomer known in the art and is commercially available as apowder. One commercially available prepolymer corresponding to the abovepolyimide oligomer is MM 9.36 available from Maverick Corporation, BlueAsh, Ohio.

As shown in Table 2, the solid powder prepolymer was blended with theliquid monomer prepolymer to form a mixture that has the MolecularWeight (“MW”) and the structural unit size (“n”) shown in the Examples.Examples 1-6 included a MW of 2100 g/mol and a structural unit size of3. Examples 7-12 included a MW of 1600 g/mol and a structural unit sizeof 2. The ratio between BTDA and BPDA was varied as shown in Table 1 andthe amount of powder added was varied, as shown in TABLE 2.

The mixture was cured at a temperature of about 600° F. (316° C.) and apressure of 200 psi for 4 hours. The glass transition temperature (“Tg”)for the cured Examples are shown in TABLE 3. The cured sample was thensubjected to a one of 2 post cures. The first post cure includesexposing the sample to a temperature of about 600° F. (316° F.) atambient pressure for 12 hours. The Tg values for the first post curedExamples are shown in TABLE 3. The second post cure includes exposingthe sample to a temperature of about 625° F. (329° C.) at ambientpressure for 12 hrs. The Tg values for the second post cured Examplesare shown in TABLE 3.

In addition to the post curing, the samples were also measured forthermal oxidative stability (TOS). The TOS for Examples 1-12 are shownin TABLE 4. Likewise, the compression strength of the samples wasmeasured after subjecting the samples to thermal cycling from roomtemperature to 550° F. (288° C.) for 380 cycles. The compression data isshown in TABLE 4.

As shown in Examples 1, 4, 7 and 10, a lower Tg and a higher TOS weightloss result from the presence of the liquid monomer mixture alone. Themixture of the liquid prepolymer component with the solid prepolymercomponent resulted in a Tg of greater than about 500° F. (260° C.) inthe cured state and a thermal oxidative stability having a TOS weightloss of less than 2.0%. In the post cured state, the Tg of Examplesreached 600° F. (316° C.) or greater.

TABLE 2 TAILORABLE POLYIMIDE RESINS NADIC END CAP Monomer Substitutionin Powder Liquid Liquid Prepolymer Formulated MW Prepolymer ComponentExample (g/mol) n = Component** Addition 1 2100 3 15%  0% 2 2100 3 15%15% 3 2100 3 15% 30% 4 2100 3 25%  0% 5 2100 3 25% 15% 6 2100 3 25% 30%7 1600 2 15%  0% 8 1600 2 15% 15% 9 1600 2 15% 30% 10 1600 2 25%  0% 111600 2 25% 15% 12 1600 2 25% 30% **percent of BTDA substituted by BPDAin liquid formulated MM 9.36 powder resin Resin MW = 936

TABLE 3 GLASS TRANSITION TEMPERATURE As Cured Tg Post Cure 1 Tg PostCure 2 Tg Example (° F.) (° F.) (° F.) 1 478 530 551 2 501 551 589 3 530576 595 4 488 531 553 5 500 556 583 6 532 579 606 7 514 552 563 8 520561 590 9 545 580 606 10 501 552 578 11 516 572 590 12 532 584 609

TABLE 4 THERMAL OXIDATIVE COMPRESSION STABILITY STRENGTH TOS WeightCompression Example Loss (%) (ksi) 1 4.83 56.95 2 1.42 89.75 3 1.6278.94 4 2.23 78.87 5 1.39 85.16 6 1.84 75.67 7 2.8 90.57 8 1.54 94.09 91.91 92.9 10 1.25 97.76 11 1.44 98.19 12 1.67 91.61

An optimized resin blend may include, in terms of molar ratio, about 2(end group component):1.35 BTDA:0.35 BPDA:1.26 phenylene diamine (mPDAand pPDA):1.44 BisM. In an exemplary embodiment, the molar ratio may be2 NE:1.35 BTDA:0.35 BPDA:0.42 mPDA:0.84 pPDA:1.44 BisM. It is envisionedthat other end capping groups may be successfully utilized in this andother exemplary formulations.

In an exemplary embodiment, some or all of the Bis M may be substitutedby bis amino phenoxy benzene (APB). The Bis M may be substituted 1 for1, maintaining the remaining molar ratios. In an exemplary embodiment,it may be desirable to increase the molar ratio of a phenylene diamine(mPDA, pPDA, or both) upon substitution of APB for Bis M. An exemplarymolar ratio formulation includes about 2 NE:about 1.35 BTDA:about 0.35BPDA:about 1.26 total (mPDA and pPDA):about 1.44 (Bis-M, APB or APB andBis-M). In an exemplary embodiment, with a substitution of at least someof the Bis M with APB, an exemplary molar ratio formulation includesabout 2 NE:about 1.35 BTDA:about 0.35 BPDA:about 1.2 total (mPDA andpPDA):about 1.5 (APB or APB and Bis M). An exemplary molar ratioincludes about 2 NE:about 1.35 BTDA:about 0.35 BPDA:about 1.7 total(mPDA and pPDA):about 1.0 (APB or APB and Bis M).

The molar ratio of total phenylene diamine (mPDA and pPDA) to APB may bein the range of from about 1.2:1.5 to about 1.7:1.0. An increase in themolar ratio of total phenyl diamine to APB may be utilized to maintainthe Tg of the cured polyimide matrix with respect to a comparablepolyimide matrix formed from a prepolymer blend without APBsubstitution.

In an exemplary embodiment a tailorable polyimide prepolymer blendincludes the end group component (e.g., NE), the dianhydride component(e.g., BTDA and BPDA) and the diamine component (e.g., mPDA, pPDA, andAPB or APB and Bis-M). Within the diamine component, the molar ratio oftotal (mPDA and pPDA) to (APB or APB and Bis-M) is in the range of about1.2-1.7 (mPDA and pPDA):about 1.0-1.5 (APB or APB and Bis-M).

In other exemplary embodiments, the molar ratio of the end groupcomponent and/or the dianhydride component may also be varied to providethe desired tailorable properties of the prepolymer blend, the curepolyimide matrix, or both.

In an exemplary embodiment, a tailorable prepolymer blend has amolecular weight of between about 1,100 to about 2,100 g/mol. In anexemplary embodiment, a tailorable prepolymer blend has a molecularweight of between about 1,200 to about 1,600 g/mol.

Exemplary prepolymer components for use in RTM processes preferably havemolecular weights of less than about 1000 g/mol. However, the desiredhigh temperature properties may not be attained from such prepolymercomponents alone. To obtained the desired properties in the curedcomposite structure, the lower molecular weight component is mixed withanother, higher molecular weight component.

In an exemplary embodiment, a preform structure is impregnated with thehigher molecular weight component. During the RTM process, the lowermolecular weight component contacts and mixes with the higher molecularweight component to form a suitable resin system. When cured, thecomposite article exhibits properties attained by utilizing the highermolecular weight component.

In an exemplary embodiment, the first and second prepolymer componentsmay independently comprise a blend of two or more resins, a monomericmixture; monomers and oligomers, and any combination thereof.

In an exemplary embodiment, the end cap component, the dianhydridecomponent, and the diamine component are present in respective amountssuch that, prior to cure, the prepolymer blend provides at least onepredetermined prepolymer blend property, and when cured under suitablecure conditions, the prepolymer blend provides a crosslinked polyimidematrix having at least one predetermined crosslinked matrix property.

For example, in an exemplary embodiment, the predetermined prepolymerblend property may be selected from a melt viscosity of the prepolymerblend (between about 1,000-20,000 cps); a molecular weight (betweenabout 1,100 to about 2,100 g/mol); a maximum cure temperature (about650° F.); suitable tack and/or drape for prepreg composites;processibility using RFI (with pressure at or below 200 psi andtemperatures at or below about 650° F.); processibility using RTM (withpressures at or below 200 psi and temperatures at or below about 650°F.), and combinations thereof.

Further, in an exemplary embodiment, the predetermined crosslinkedmatrix property may be selected from a thermal oxidative stability (lessthan 4% weight loss when exposed to 555° F. and 50 psi for 1000 hours);a glass transition temperature (at least about 450° F. or at least about525° F.) a void content (less than about 3%), room temperature tensilestrength (at least about 100 ksi); room temperature compression strength(at least about 80 ksi); room temperature inplane shear (at least about8 ksi), and combinations thereof.

In an exemplary embodiment, an article is formed from any of theexemplary tailorable polyimide prepolymer blends. The article may be apowder, a neat resin, a coating material, a film, an adhesive, a fiber,a composite, a laminate, a prepreg, a part, and combinations thereof.

Thus, embodiments disclosed herein provide processes suitable forforming polyimide composite articles. Exemplary processes include resintransfer molding (RTM) and resin infusion (RI). In an exemplaryembodiment, the molar ratios of the various components may be varied toprovide melt viscosities and molecular weights to provide RTM or RIprocessible prepolymer blends. In an exemplary process, a preformstructure including reinforcing materials is tackified with a firstprepolymer component of the polyimide resin system. A second prepolymercomponent of the polyimide resin system is forced into contact with thetackified preform. During the cure process, the first and secondprepolymer components of the polyimide system intermingle and react toform the final crosslinked resin matrix.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

1. A method comprising: tackifying a preform structure with a firstprepolymer component; contacting the tackified preform structure with asecond prepolymer component, wherein a polyimide resin system iscomprised of the first prepolymer component and the second prepolymercomponent; and curing the polyimide resin system under suitableconditions such that the first and second prepolymer components mix andreact to provide a polyimide composite structure.
 2. The methodaccording to claim 1 including providing the preform structure, whereinthe preform structure comprises reinforcing fiber having a form selectedfrom woven fabric, non-crimp fabric, unitape, fiber film, uniaxial,braided, and combinations thereof.
 3. The method according to claim 1wherein tackifying the preform structure includes contacting the preformstructure with the first prepolymer component having at least onephysical property restricting use of the first prepolymer component in aresin infusion (RI) or resin transfer molding (RTM) technique.
 4. Themethod according to claim 1 wherein tackifying the preform structureincludes contacting the preform structure with the first prepolymercomponent comprising at least one of a monomeric blend or a reactionproduct of an end-capping component, at least one dianhydride orderivative thereof, and at least one diamine.
 5. The method according toclaim 1 wherein tackifying the preform structure includes contacting thepreform structure with the first prepolymer component comprising atleast one of a monomeric blend or a reaction product of norbornene2,3-dicarboxylic acid (NE); 3,4,3′,4′-benzophenonetetracarboxylicdianhydride (BTDA); 1,3-phenylenediamine (mPDA); and4,4′-(1,3-phenylene-bis(1-methylethlidene) bisaniline (Bis-M).
 6. Themethod according to claim 1 wherein tackifying the preform structureincludes contacting the preform structure with the first prepolymercomponent comprising at least one of a monomeric blend or a reactionproduct of norbornene 2,3-dicarboxylic acid (NE);3,4,3′,4′-benzophenonetetracarboxylic dianhydride (BTDA);3,3′4,4′-biphenyl-tetracarboxylic dianhydride (BPDA); paraphenylenediamine (pPDA); and 4,4′-(1,3-phenylene-bis(1-methylethlidene)bisaniline (Bis-M).
 7. The method according to claim 1 whereincontacting the tackified preform structure with the second prepolymercomponent includes utilizing a processing technique selected from resininfusion (RI) or resin transfer molding (RTM).
 8. The method accordingto claim 1 wherein the second prepolymer component includes at least oneof a monomeric blend or a reaction product of an end group component, atleast one dianhydride or derivative thereof, and at least one diamine.9. The method according to claim 1 further including: providing thesecond prepolymer component in at least one form selected from a moldingcompound and a film material.
 10. A method including: tackifying apreform structure with a first prepolymer component comprising at leastone of a monomeric blend or a reaction product of an end-cappingcomponent, at least one dianhydride, and at least one diamine;contacting the tackified preform structure by a technique selected fromresin infusion (RI) and resin transfer molding (RTM) with a secondprepolymer component being different from the first prepolymercomponent, wherein the second prepolymer component includes at least oneof a monomeric blend or a reaction product of an end-capping component,at least one dianhydride or derivative thereof, and at least onediamine, wherein a polyimide resin system is comprised of the firstprepolymer component and the second prepolymer component; and curing thepolyimide resin system under suitable conditions such that the first andsecond prepolymer components mix and react to provide a polyimidecomposite structure.
 11. The method according to claim 10 whereintackifying the preform structure includes providing a sufficient amountof the first prepolymer component to mix and react with the secondprepolymer component to provide the polyimide composite structure withat least one predetermined property.
 12. A method comprising: providinga polyimide resin system comprising an amount of a first prepolymercomponent and an amount of a second prepolymer component; providing apreform structure comprising reinforcing material; tackifying thepreform structure with at least a portion of the first prepolymercomponent; contacting the tackified preform with the second prepolymercomponents and a remainder of the first prepolymer component; and curingthe tailorable polyimide resin system under suitable conditions suchthat the first and second prepolymer components mix and react to providea polyimide composite structure.